It is well known, when measuring time of occurrence of input signals, that timing discriminators are typically required in high resolution time measurement techniques. It is desired that these discriminators accurately mark the time arrival of the input signals without respect to their intensity or their specific shapes. Constant-fraction discriminators or leading-edge discriminators are typically used. In the event of a bi-polar input signal, zero-crossing discriminators may be used.
The circuit of the constant-fraction discriminators defines two paths. The first is an attenuator path and the second a delay path. The attenuator path acts to reduce the amplitude of the input signals by a selected amount. The delay path does not alter the amplitude of the signal, but delays the arrival of the signal to a differencer. The differencer will then essentially subtract the attenuated signal from the delayed signal. The result will be a wave form having an initial negative value and then crossing the x-axis (y=zero) and going positive. This crossing is referred to as the zero-crossing. Signals that define similar waveforms but have differing amplitudes will each define substantially the same zero-crossing. Therefore, the discriminator is amplitude insensitive.
Delay lines of the constant-fraction discriminator which are typical of the prior art include the use of a selected length of cable. The length of cable simply delays the signal due to the length of travel the signal must traverse to reach the differencer. The length of cable is selected by determining the delay per linear unit and dividing that number into the desired delay time. A delay of 7.5 ns using a cable having a 1.5 ns per ft delay would obviously require 5.0 ft of cable. The cable is typically coiled up to minimize the required space. However, it is well known that this type of delay line usually requires more space than is desired. Further, if the delay time is to be altered, a cable of a length corresponding to the desired delay time must be installed. Therefore it is evident that a supply of delay lines, each defining a unique length, must be maintained.
U.S. Pat. No. 4,443,768 issued to C. H. Nowlin on Apr. 17, 1984 discloses a filter for converting an input pulse having random amplitudes and non-zero rise times to a bi-polar output pulse having a zero-crossing time that is independent of the rise time and the amplitude of the input pulse. In order to accomplish the desired output, the Nowlin filter replaces the delay line with a differentiator. The circuit operates by differencing an attenuated version of the signal with a differentiated version. The requirement of timing along the leading edge of a linear-edge input signal is also necessary for amplitude-rise-time-compensated timing using the delay-line constant-fraction discriminator. The Nowlin shaping-signal zero-crossing time is insensitive to input-signal amplitude for non-linear-edge input signals of fixed, arbitrary shape.
The simplest implementation of the Nowlin circuit consists of a single-pole high-pass filter, acting as an approximate differentiator, combined with an attenuation and differencing circuit. The leading edge of the input signal is described in the time and Laplace domain as: EQU V.sub.in (0.ltoreq.t.ltoreq.t.sub.r)=(V.sub.inpk /t.sub.r)t
and EQU V.sub.in (s)=(V.sub.inpk /t.sub.r)(1/s.sup.2)
where V.sub.inpk is the peak input-signal amplitude (occurring at t=t.sub.r) and t.sub.r is the input-signal linear-edge rise-time. The transfer function for the Nowlin constant-fraction discriminator is given by: EQU H.sub.cf (s)=f{(1-st.sub.d)((1-f)/f)}/(1+St.sub.d)
where f is the attenuation gain, and t.sub.d is the time-constant associated with the single-pole high-pass network.
The shaping signal is represented in Laplace notation as the product of the input-signal and the shaping network Laplace expressions, which are the latter two equations, respectively. The time-domain shaping signal is found from the inverse Laplace transform of this product and is given by: EQU V.sub.cf (0.ltoreq.t.ltoreq.t.sub.r)=(V.sub.inpk /t.sub.r){ft-t.sub.d (1-e.sup.-t/td)}.
This shaping signal starts at the origin (a value of zero at time t=0) and must go negative before making a positive-going zero crossing. An initial negative signal swing, or underdrive, requires that the fraction f be less than unity, and a positive-going crossing requires that f be greater than zero. Further, the positive-going zero crossing must occur during the input-signal rise time for linear-edge rise-time-insensitive timing, thereby placing a further restriction on the fraction value. The range of acceptable values are described by: EQU 0&lt;f&lt;1
and EQU f.ltoreq.(t.sub.d /t.sub.r)(1-e.sup.-tr/td)
where the fraction and single-pole high-pass filter time constant must each be selected based on the smallest expected input-signal rise time. The zero-crossing time occurs when v.sub.cf as given above is equal to zero and is found by equating {ft-t.sub.d (1-e.sup.-t/td)} to zero and solving for t. Therefore, it is obvious that the zero-crossing time is insensitive to the input-signal amplitude (V.sub.inpk) and the rise time (t.sub.r).
Though the device of the '768 patent provides a timing-shaping circuit which is amplitude- and rise-time-insensitive, the implementation thereof is not easy in some integrated circuitry. For example, the circuit using a single-pole CR high-pass filter (an approximate differentiator) requires a capacitor with both terminals isolated from ground. Such a capacitor may not be easily implemented in some integrated circuitry.
Other types of discriminators are described by C. H. Nowlin in "Low-Noise Lumped-Element Timing Filters with Rise-Time Invariant Crossover Times", Rev. Sci. Instrum., Vol 63, No. 4, pp. 2322-2326, April, 1992; and B. T. Turko, et al., in "A Precision Timing Discriminator for High Density Detector Systems", IEEE, pp. 711-715, 1992.
The Nowlin article describes lumped-element filters which do not include the use of delay lines or their equivalents. Nowlin is primarily directed toward the theoretical aspects of the relationships between noise-caused timing variances, crossover time, and waveform.
Turko, et al. describes a zero-crossing circuit consisting of a voltage comparator and a single RC integrating network. The circuit shown by Turko, which will sense the peak of a narrow pulse, uses a single-pole low-pass filter with a fraction value of 100%. The fraction value is calculated by dividing the attenuated signal gain by the delayed signal gain. There can be no zero-crossing in this circuit until the input reaches a peak and begins falling as there is no attenuated signal. The response of the Turko circuit is that of a single-pole CR high-pass filter, an approximate differentiator, that approximately senses the peak of the input signal where the input signal slope is zero.
Therefore, it is an object of this invention to provide a means for marking the time arrival of input signals without respect to their intensity or their specific shapes.
Another object of the present invention is to provide such a marking means by using a constant-fraction discriminator which may be contained with integrated circuitry for cost and storage efficiency.
Still another object of the present invention is to provide a constant-fraction discriminator which is insensitive to input signal amplitude and rise time.
Yet another object of the present invention is to provide a constant-fraction discriminator which uses low-pass filter integration techniques or an all-pass filter to achieve a delayed signal for comparison with an attenuated signal.